# Heat Engines And The Second Law Of Conservation Of Energy

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1.1 What is a heat engine? In thermodynamics, a heat engine is a device/system that uses heat to do work, i.e. it converts heat or thermal energy to mechanical energy. 1.2 How does a heat engine work and its operating principles?
For a heat engine to work, it must operate in cycles.
Firstly, heat is added into the system. Let the heat added into the system be represented by [Qh]. After that process, some of the energy that were added into the system will be used to do work e.g the materials expanding or increasing in temperature
Let the work done be [W]. After that, the rest of the heat will then be removed at a cold temperature.
Let the heat removed be represented by [Qc]. If we apply the law of Conservation of Energy, we
The second law of thermodynamics states that even under ideal conditions, no heat engine can convert all the energy it receives to do useful work, some energy will be lost, and each time a system goes through a thermodynamic process, the system can never completely return to exactly the same state it was before (irreversibility). For example, if a hot object is placed next to a cold object, the cold object will become warmer and the hot object will be cooler until a thermal equilibrium is reached. However, heat will still be lost to the surroundings. It is common knowledge that heat, of its own volition, does not transfer from a cold medium to a warmer medium. It is therefore impossible to construct a refrigerator that operates without an input of work. So for a refrigerator to work it requires an input of energy, Win.
(From text
While refrigerator will take in heat as energy. Another difference is that in a heat engine, heat travels from the cold reservoir to the hot reservoir, shown below in Fig 2.6a. While in a refrigerator heat travels from the hot reservoir to the cold reservoir, shown above in Fig 2.5a.
Image
Fig 2.5a Typical Heat Engine Text BoxImage
Fig 2.5b: explanation of more uses of the heat engines.
2.6 How a heat engine can be used to drive a refrigerator:
In a refrigerator, the heat engine is reversed. This allows the refrigerator to cool and maintain the temperature of an object to lower than the atmospheric or room temperature.

Image
Fig 2.6a Reverse Heat Engine for Refrigerators

The temperature of the gas in a refrigerator must be lower than that of the temperature of the cold reservoir before it can extract heat QC from the cold reservoir. Heat energy is thus being transferred from the cold reservoir into the colder gas.

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